The invention relates to methods and apparatus for making crystals. While not wishing to be bound to a particular group of crystals, it is convenient in the following discussion to focus on a particular group of crystals. One group of crystals of interest is optical fluoride crystals. Single-grained optical fluoride crystals are useful in applications requiring transmission at short wavelengths, e.g., in the vacuum ultraviolet region. At present, CaF2 crystal is the most viable lens material for 157-nm microlithography.
Optical fluoride crystals as well as other types of crystals can be grown in two steps. The first step is a pre-melt step in which crystal raw material in powder or granular form is melted and then rapidly cooled into a solid body, which herein will be referred to as a crystal pre-melt. The second step is a growth step in which the crystal pre-melt is melted and then used to grow one or more crystals.
Single-grained crystals are commonly grown using the Bridgman-Stockbarger process. As illustrated in FIG. 1A, the Bridgman-Stockbarger process takes place in a vertical furnace 100 having an upper zone 102 and a lower zone 104. A middle or thermal gradient zone 106 is defined between the upper zone 102 and the lower zone 104 by making the temperature of the upper zone 102 higher than the temperature of the lower zone 104. The growth step starts with loading of a crystal pre-melt 108 into a crystal growth crucible 110. With the growth crucible 110 in the upper zone 102, the upper zone 102 is then heated to a temperature sufficient to melt the crystal pre-melt 108. After melting the crystal pre-melt 108, the growth crucible 110 is lowered from the upper zone 102, through the thermal gradient zone 106, into the lower zone 104. As shown in FIG. 1B, as the growth crucible 110 passes through the thermal gradient zone 106, a phase transition occurs inside the molten crystal pre-melt 108a, creating a crystal front 112. The crystal front 112 propagates inside the growth crucible 110, within the molten crystal pre-melt 108a, as long as the growth crucible 110 continues to move downwardly into the lower zone 104.
Returning to FIG. 1A, the crystal pre-melt 108 is made by subjecting relatively pure crystal raw material in powder or granular form to a pre-melt step. The main purpose of the pre-melt step is to increase the bulk density of the crystal raw material so that the volume of the crystal growth furnace can be used efficiently. For example, synthetic CaF2 powder has an apparent density of 1.1 g/cm3, and crystals grown from CaF2 powder have an apparent density that is close to the theoretical density of CaF2, i.e., 3.18 g/cm3. Thus, if synthetic CaF2 powder is used directly as the crystal raw material, two-thirds of the volume of the crystal growth furnace will not be used, or at least, not used efficiently. Through a pre-melt step, the apparent density of the CaF2 raw material can be increased to approximately 2.2 g/cm3, which would be much closer to the density of the grown CaF2 crystal.
The pre-melt step typically includes a treatment step in which oxide impurities are scavenged from the crystal raw material prior to melting the crystal raw material. Commonly, the treatment step involves mixing a solid fluorinating agent, e.g., PbF2, ZnF2, or polytetrafluoroethylene (PTFE), with the crystal raw material and then heating the mixture to the melting point of the crystal raw material. At this temperature, the solid fluorinating agent reacts with oxides in the crystal raw material to form volatile gases that escape from the crystal raw material. However, use of solid fluorinating agent to scavenge oxide impurities has a drawback in that impurities such as Pb2+, Zn2+, or C can remain in the crystal raw material after treatment. For optical fluoride crystals, the presence of these impurities, even at trace levels, can result in absorption bands that are detrimental to transmission at wavelengths below 200 nm.
Recently, gaseous fluorinating agents, such as CF4, HF, SF6, and BF3, have been proposed as alternatives to solid fluorinating agents in scavenging oxide impurities. With gaseous fluorinating agents, the risk of leaving harmful impurities such as Pb2+, Zn2+, or C in the grown crystal is relatively low or non-existent. However, use of gaseous fluorinating agents in scavenging oxide impurities in conventional settings poses various challenges. For example, the furnace and furnace elements, e.g., crucibles, thermocouples, and resistors, used in the pre-melt step are typically made of pure graphite or other materials that are susceptible to corrosion by the gaseous fluorinating agents, especially in the presence of oxygen atmosphere and/or at elevated temperatures. For example, CF4 attacks thermocouples at temperatures above 1100° C. Further, pure graphite is porous and tends to outgas. Outgassing of H2O, CO2, or CO from the graphite parts may contribute to contamination of the crystal pre-melt after treatment.
From the foregoing, there is desired a practical method of making a crystal pre-melt from crystal raw material involving use of a gaseous fluorinating agent as an oxide scavenger.